Nonpolar liquids, penetration

If the principal cohesive forces between solute molecules are London forces, then the best solvent is likely to be one that can mimic those forces. For example, a good solvent for nonpolar substances is the nonpolar liquid carbon disulfide, CS2-It is a far better solvent than water for sulfur because solid sulfur is a molecular solid of S8 molecules held together by London forces (Fig. 8.19). The sulfur molecules cannot penetrate into the strongly hydrogen-bonded structure of water, because they cannot replace those bonds with interactions of similar strength. 

Several studies have been concerned with the penetration of liquids into latewood and earlywood (JJ, 16-23). Under atmospheric pressure, the penetration of nonpolar liquids into softwood latewood may be caused, in part, by capillary action in the very small lumens and passage through unaspirated pit membranes. In aspirated earlywood this penetration would not occur. Penetration of nonpolar liquids may also be through drying checks in the thick latewood cell walls. As the temperature and pressure of the liquid are raised, penetration of polar liquids in earlywood would be expected to increase because of softening of the pit structure and displacement of the pit membrane. Because the cell wall of earlywood is thinner than that of latewood, penetration into earlywood walls would be quicker and facilitated by swelling. Incrustation occurs in the pit membranes of southern pine latewood (24) this would retard liquid penetration. 

If crude oil or oil products penetrate the aeration zone (Figure 2.69), their most volatile components pass into composition of the subsurface gas, and most soluble - into composition of ground water. However, their greater portion is preserved either as residual saturation in the rock (groxmd) or as cumulation at the level of ground water (if they are lighter than water). Finally, all nonpolar components tend to balanced distribution between water, subsurface gas, nonpolar liquid and the rock. 

Based on these simulations, it was proposed that inserting nonpolar peptides into nonpolar liquids and, possibly, membranes requires simultaneous folding and penetration. Some penetration into hexane is needed to initiate folding. As... 

SFE is used mainly for nonpolar compounds [e.g. polychlorinated biphenyls (PCBs)]. Typically, small aliquots of soil (0.5-10 g) are used for extraction. The extraction solvent is a supercritical fluid, most commonly carbon dioxide, which has properties of both a liquid and gas. The supercritical fluid easily penetrates the small pores of soil and dissolves a variety of nonpolar compounds. Supercritical carbon dioxide extracts compounds from environmental samples at elevated temperature (100-200 °C) and pressure (5000-10 000 psi). High-quality carbon dioxide is required to minimize... 

The permeability of the skin to a toxic substance is a function of both the substance and the skin. The permeability of the skin varies with both the location and the species that penetrates it. In order to penetrate the skin significantly, a substance must be a liquid or gas or significantly soluble in water or organic solvents. In general, nonpolar, lipid-soluble substances traverse skin more readily than do ionic species. Substances that penetrate skin easily include lipid-soluble endogenous substances (hormones, vitamins D and K) and a number of xenobiotic compounds. Common examples of these are phenol, nicotine, and strychnine. Some military poisons, such as the nerve gas sarin (see Section 18.8), permeate the skin very readily, which greatly adds to then-hazards. In addition to the rate of transport through the skin, an additional factor that influences toxicity via the percutaneous route is the blood flow at the site of exposure. 

Capillary penetration experiments. Using a series of nonpolar and polar probe liquids, the solid surface tension of unmodified and modified particles was evaluated by capillary penetration experiments [35,36], The strategy is based on a modified Washburn equation ... 

Extensive experimental, theoretical, and modeling work has been directed at revealing details of the complex interface between the mobile and stationary phases (Beck and Klatte, 2000 Slusher and Mountain, 1999 Zhang et al., 2005). Current debates are centered on the issue whether retention can be correctly understood as bulk liquid-phase partitioning or an adsorption process. The simulation work has shown that it is really neither of these two extremes the interface possesses specific ordering features that present a non-bulk environment, yet nonpolar solutes do penetrate significantly into the... 

One of the problems with using a liquid as the extraction solvent is its removal when the extraction is finished. The most recent way to eliminate this problem is to use a supercritical gas, COj being the gas of choice at the moment. A gas in the supercritical state has solvent properties comparable to a liquid but it is less viscous, so it can penetrate the sample faster. When the extraction is complete, the pressure is released, and the gas evaporates away from the extracted components. CO2 is nonpolar so more polar compounds such as methanol are sometimes added in small amounts. This exceWeni supercritical fluid extraction (SEE) technique is described in Chapter 13. 

Colloid chemists commonly measure surface area by the adsorption of N2 gas. The adsorption is conducted in vacuum and at temperatures near the boiling point of liquid nitrogen (—196° C). The approach is based on the Brunauer-Emmett-Teller (BET) adsorption equation, and has been adapted to a commercially available instrument. Unfortunately, the technique does not give reliable values for expansible soil colloids such as vermiculite or montmorillonite. Nonpolar N2 molecules penetrate little of the interlayer regions between adjacent mineral platelets of expansible layer silicates where 80 to 90% of the total surface area is located. Several workers have used a similar approach with polar H2O vapor and have reported complete saturation of both internal (interlayer) and external surfaces. The approach, however, has not been popular as an experimental technique. 

The molecular mechanism of ion transfer across liquid-liquid interfaces is remarkably similar to the mechanism of unassisted ion transport across membranes, recently studied by molecular dynamics simulations. The study revealed that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. As can be seen in Figure 14, these defects are quite similar to water fingers observed during... 

Fifty-six isothermal data sets for vapor-liquid equilibria (VLB) have been used for 15 polymer-HSolvent binaries, 11 copolymer-nsolvent binaries and for 30 polymer-polymer-solvent ternaries to study compatibility of polymer blends. The equilibrium solubility of a penetrant in a polymer depends on their mutual compatibility. Equations based on theories of polymer solution tend to be more successful when there is some kind of similarity between the penetrant and the monomer repeat unit in the polymer, e.g., for nonpolar penetrants in polymers which do not contain appreciable polar groups. Expected nonideal behavior has been observed for systems containing hydrocarbons and poly(acrylonitrile-co-butadiene). The role of intramolecular interaction in vapor-liquid equilibria of copolymer-nsolvent systems is well documented for poly(aciylonitrile-co-butadiene) that have higher affinity for acetonitrile than do polyaciylonitrile or polybutadiene. 

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